The invention relates generally to a heat fusing member and more particularly to a fuser roll having a fluoroelastomer surface for applying heat and pressure to fix toner to recording paper. The fluoroelastomer surface permits toners to be fixed to the recording paper without offset and can withstand continuous exposure to high temperature, silicone oils, toners, toner additives and paper product residue without unacceptable physical degradation.
In general, when forming images by xerographic processes, an image formed of a heat fusible powdered toner is selectively disposed on a web-like surface of a recording medium, such as paper by electrostatic forces. The toner is fixed to the paper by applying heat and pressure by a fuser member such as a heated roller during a fusing process. The toner powders are commonly a mixture of thermoplastic and thermosetting resins having amorphous carbon and magnetic particles incorporated therein and are conventionally fused by direct contact with a fuser roll to temperatures between about 200.degree. to 400.degree. F.
The actual temperature range suitable is referred to as the "fusing window." Fusing window, TW=T.sub.off -T.sub.min, wherein T.sub.off is the Hot Offset temperature and T.sub.min is the minimum fusing temperature. Hot Offset is the temperature at which coercive forces within the molten toner layer are less than the adhesive forces between the toner and roller surface. T.sub.min is the minimum temperature at which toner can be acceptably fixed to the recording paper. This temperature range is dependent on the roll materials, the type of toner, release agents and the pressure. What is important is that the toner be fixed without "offset" occurring. For commercial application a fusing window of at least 30.degree. F. is utilized in some machines, but the larger the better. Thus, a 60.degree. F. fusing window is ideal and 100.degree. F. is particularly desirable.
The toner image is fused to the recording paper by heating above its softening point and applying pressure to force the softened toner into the interstices of the paper fibers. As thermoplastic resin toner cools, it becomes fixed to the recording paper. Thermosetting resin toners fix to the recording paper by a cross-linking mechanism.
The fusing process is conventionally performed by feeding a recording medium having the toner thereon between the nip where two mated rollers meet. One or both of the rollers are heated internally so that the surface temperature of the rollers will be above the softening point of the resinous carrier of the toner. The recording medium with the toner image thereon is fed between the two rollers which press towards each other to apply direct heat and pressure to the toner image. The amount of pressure and the length of time that the toner is heated determine the degree of fusing.
Conventional fuser roller systems have drawbacks. Softened toner generally has an affinity for the surface of the fuser roll it contacts. When toner adheres to the surface of the fuser roll, it can be unintentionally deposited on an unselected portion of the recording medium during the next rotation of the roller. This phenomenon is referred to as offset.
To prevent offset, a thin coating of a release agent such as a polysiloxane fluid is commonly spread over the surface of the fuser roll which contacts the surface with the toner image. The polysiloxane fluid reduces the surface free energy of the roller surface and decreases the affinity of the toner for the roller. However, the release agent is transferred to the surface of the recording medium during fusing of the image. This can interfere with the ability to write on the surface of the recording medium. Furthermore, polysiloxane fluid causes premature failure of certain types of roll covering materials, because it is absorbed into the surface of the roll covering. This reduces fuser roll wear resistance and causes swelling of the roll covering which can lead to an uneven pressure distribution between the two rollers and non-uniform fusing resulting in poorer printing quality.
Fuser rolls are commonly made with a surface material of one of three classes of materials: polyfluorocarbon resins, polysiloxane elastomer and polyfluorocarbon elastomers. Each of these three classes of materials exhibit certain inadequacies although each have an appropriate level of heat resistance and thermal stability.
Polyfluorocarbon resins have drawbacks because they lack sufficient flexibility and elasticity. This adversely affects copy quality because the surface of the fuser roll is harder than the softened toner and is not deformed by the toner. It therefore can displace the toner image and lead to a non-uniform image loss and reduced image purity.
Polysiloxane elastomers are adequately flexible and elastic and lead to high quality fused images. However, after an unacceptably low number of copies are produced, the self release properties of the roll degrade and offset begins to occur. Using a polysiloxane fluid in connection with polysiloxane elastomer rollers enhances the ability of the rollers to release toner, but shortens the roller life due to silicone oil absorption.
Polyfluorocarbon elastomers commonly have unacceptable toner
release properties resulting from their high surface tension of 35-37 nMn. Release agent fluid is necessary. Surface tension values for several fuser roll materials are set forth below in Table I.
TABLE I ______________________________________ Surface Tension of Fuser Roll Materials Material Surface Tension nMn ______________________________________ Polyfluorocarbon Resins Polyhexafluoropropylene (PHFP) 16.2-17.1 Polytetrafluoroethylene (PTFE) 18.0-18.5 Polyvinylidene fluoride (PVF.sub.2) 21-22 Polysiloxane Oil 28-29 Polyfluorocarbon Elastomer 35-37 ______________________________________
U.S. Pat. Nos. 4,257,699, 4,264,181 and 4,272,179 describe various fuser roll constructions designed to solve many of the aforementioned inadequacies. These fuser rolls have a core and at least two elastomer layers disposed on the core. Preferred elastomers are fluoroelastomers containing residual metal compounds with at least the outer elastomer layer including additional metal-containing filler dispersed therein. A polymeric release agent having mercapto functional groups is applied to the surface of the fuser roll. The metal-containing filler in the outer elastomer layer must be present in an amount sufficient to interact with the polymeric release agent upon the working surface of the fuser roll to yield a release "film". This film prevents the thermoplastic resin toner from contacting the elastomer material itself. The film must have surface energy that is less than the surface energy of the toner at operating temperatures. While this construction is satisfactory, it has drawbacks. The silicone fluid having mercapto functional groups polymeric release agents described therein are expensive and interfere with the ability to write on the paper after fusing. They present an unpleasant odor in the office environment, are significantly more expensive and frequently contaminate internal and external surfaces of the copying equipment and the copier office environment.
Accordingly, it is desirable to provide an improved fuser system which overcomes the shortcomings of the conventional fuser systems described above.